This invention relates generally to electromagnetic flowmeters, and more particularly to a flangeless flowmeter having a cylindrical housing and whose components are integrated to form a highly compact, low-cost unit that may be readily installed in a flow line between the flanged ends of the upstream and downstream pipes, the flowmeter including relatively thin electromagnet coils which conform to the inner surface of the housing.
Magnetic flowmeters such as those disclosed in U.S. Pat. Nos. 3,695,104; 3,824,856; 3,783,687 and 3,965,738, are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because the instrument is free of flow obstructions, it does not tend to plug or foul.
In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube through which the fluid to be metered is conducted and to the transverse axis along which the electrodes are located at diametrically-opposed positions with respect to the tube. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.
The typical commerically-available magnetic flowmeter is provided with mounting flanges at either end thereof. The meter is interposed between the upstream and downstream pipes of a fluid line, each pipe having an end flange. The mounting flanges on the meter are bolted to the flanges of line pipes. It is, of course, essential that the circle of bolt holes on the mounting flanges of the meter match those on the pipe flanges.
In a magnetic flowmeter, the flow tube is subjected to the same fluid pressure as the line pipes. The flow tube must therefore be of a material and of a thickness sufficient to withstand this pressure, even though the strength of the flow tube is unrelated to its measuring function. This design factor contributes significantly to the cost of a standard meter. Existing meters are made up of components that must be assembled are generally of substantial size and weight and quite expensive to manufacture.
In order to provide a compact and readily installable electromagnetic flowmeter whose weight and dimensions are substantially smaller than existing types, the above-identified related patent applications disclose highly compact flangeless flowmeters which, despite their reduced volume and weight, are capable of withstanding high fluid pressures, the flowmeters operating efficiently and reliably to accurately measure flow rates.
The flangeless flowmeters disclosed in these related cases are interposable between the flanged ends of upstream and downstream line pipes to meter fluid passing through the line. In one preferred embodiment, the meter is constituted by a ferromagnetic ring within which a pair of electromagnet coils is supported at opposed positions along a diametrical axis normal to the longitudinal axis of the ring, the longitudinal axis passing through the central flow passage of an annular pressure vessel which is formed of high strength insulating material and is molded within the ring to encapsulate the coils as well as a pair of electrodes disposed at diametrically-opposed positions with respect to the passage along a transverse axis at right angles to the coil axis to define a unitary structure. The unit is compressible between the end flanges of the pipes by bridging bolts that pass through bore holes in the pressure vessel or which lie outside of the ring to encage the unit.
My related patent application Ser. No. 075,037 (U.S. Pat. No. 4,253,340) discloses a flangeless flowmeter interposable between the flanged ends of upstream and downstream pipes in a fluid line for metering fluid passing therethrough, the meter including a non-magnetic metal spool of high mechanical strength which functions as a flow conduit and also renders the meter capable of withstanding high compressive forces as well as fluid pressure. This non-magnetic metal spool is surrounded by a ferromagnetic housing which acts as a mold for potting the inner chamber defined between the spool and the housing and thereby sealing the components contained therein. The housing also serves as the magnetic flux return path for the electromagnets supported thereby.
The housing in this related case is formed by complementary half-pieces which include end plates that join the corresponding ends of the spool to define the inner chamber. Integral with the half-pieces are two magnet cores which extend at diametrically opposed positions along an axis normal to the longitudinal axis of the cylindrical housing, the cores being surrounded by coils to define solenoid-type electromagnets. Mounted on the spool at diametrically opposed positions along a transverse axis at right angles to the core axis are two electrodes.
Thus in my prior arrangement, the electromagnetic field is produced by a pair of cored solenoids occupying diametrically-opposed positions with respect to the longitudinal axis of the flow conduit, the cores being magnetically coupled by the ferromagnetic housing. The air gap which exists between the pole faces of these cores depends on the diameter of the flow conduit: the larger the diameter, the bigger the gap. An arrangement of this type is appropriate to flowmeters having flow conduits of small diameter such as one inch; but, as will now be explained, it is unsuitable for larger diameters--that is, diameters of two, three and four inches and greater.
The reason why cored solenoids are inappropriate in larger diameter flowmeters will be clear from the basic flowmeter equation: EQU e.sub.flow= BLv
wherein:
e.sub.flow is the flowmeter signal intensity; PA1 B is the magnetic field strength; PA1 L is the diameter of the flow conduit; PA1 v is the flow velocity.
For a constant e.sub.flow /v relationship, B is inversely proportional to L. Hence, as the flow conduit diameter L increases with larger meters, one requires a reduction in magnetic field strength, this being accomplished by fewer ampere turns which results in a thinner electromagnet coil. The use of unduly thick coils for a given meter diameter would obviously be wasteful of power, resulting in excessive heat and adding unnecessarily to the cost of the meter.
With larger diameter flow conduits, the air gap becomes the predominant influence on the system permeability, and the effect of cores or pole pieces on this factor becomes less significant. Thus if a meter has a 12 meter diameter flow conduit, the appropriate coil therefor is only a quarter of an inch thick. In this context, a one quarter inch thick pole piece would have a negligible effect on the permeability of the system, and is therefore unnecessary except possibly as a post on which to mount the coil.